FOCUS: IRRIGATION & WATER MANAGEMENT Still lots to learn from soil moisture monitoring

KAREN DAVIDSON

Precision agriculture and irrigation start with careful observation of each agricultural field during the growing season with a variety of sensors below, on, and far above the ground.

That’s the day-to-day business of Willemijn Appels, the senior research chair, Mueller Irrigation Group at Alberta’s Lethbridge College. She and her team have just completed a four-year study of the potential benefits of variable rate irrigation (VRI) for southern Alberta potato growers. The 19 sites range in topography and soil types from Lethbridge to Bow Island across two irrigation districts.

All modern pivots have electronic control panels, however variable rate pivots have the ability to turn sets of sprinklers on/off based on their position in the field. Data obtained by sensors need to be analyzed to create an irrigation prescription. Growers can upload their prescriptions to the control panel, which then applies water accordingly.

“We use existing and new sensors to obtain a suite of observations to get a better, timely and quantitative picture of soil water availability and crop water demand during the growing season,” says Appels.

Examples of tools are: in-situ soil moisture sensors, weather stations, portable probes, a UAV (drone), remote sensing imagery.

“In addition to hands-on experimenting, we develop plant-soil-water models to simulate the effects of new irrigation techniques and adaptive scheduling on yield of the crops found in southern Alberta,” she adds.

The use of computer models allows an analysis of these effects over the entire range of soil types and topographic features found in the region. Findings from the lab and field experiments are used to calibrate the models. The results of the computer simulations and the map visualizations help guide field experimental programs. They are also inputted to financial calculations to evaluate return on investment of variable rate irrigation systems.

Findings

Appels’ findings confirm the instincts of local growers. There wasn’t much consistent variability of soil moisture in fields in this region, which limits the benefits to be gained with variable rate irrigation. In the Bow Island area, where study fields were more “hilly” by prairie standards, there were larger differences in water availability in the root zone for longer parts of the growing season.

Most farms are doing a very good job of irrigating and crop water demands were generally met without the need for implementation of VRI.

As Appels says, “We found that soil moisture levels in potato hills varied less with topography than soil moisture levels in the furrows. VRI may therefore be useful to guarantee field accessibility throughout the growing season. However, this also means that you cannot rely on static maps for VRI management.”

“All of this to say that there’s a relatively small opportunity to boost yields by implementing variable rate irrigation,” says Appels. “There may be an 8-9 per cent potato yield difference which can be attributed to soil moisture – and that may be worthwhile in a high-value crop, especially when a field contains more ‘hilly’ features. But these statistics imply that there is not a one-size-fits-all approach to the implementation of VRI in southern Alberta.”

The next question may be what criteria are needed to set up irrigation zones. That’s what Appels will be pursuing in 2023.